Iowa U.S. Senate race: State Sen. Joni Ernst has taken the lead in the polls in the crowded GOP primary against former District Attorney Matt Whitaker. Ernst gained national prominence after an ad entitled "Squeal," in which she talked about castrating pigs. Both are looking to replace U.S. Sen. Tom Harkin (D-IA).

Iowa U.S. House District 3: Six GOP candidates are facing off for the seat being vacated by Rep. Tom Latham (R). But a recent poll found that none of the candidates appeared close to getting 35 percent of the vote. If that threshold is not reached in Tuesday's primary, the GOP nominee will be chosen at a convention of GOP activists.

Mississippi U.S. Senate race: State Sen. Chris McDaniel backed by the Tea Party, is challenging Republican U.S. Senator Thad Cochran. While the polls show this race to be close, the Tea Party challenger has been swept up in a scandal which saw several of McDaniel's supporters arrested in a scheme to exploit Cochran's wife, who has dementia.

California Governor race: Republican conservative, state Assemblyman Tim Donnelly has a strong anti-immigration stance which could drive Democratic voters to the polls in November. Former Assistant Treasury Department Secretary Neel Kashkari has put more than $2 million of his own cash into his campaign and been endorsed by Romney, former Florida Gov. Jeb Bush (R) and former Secretary of State Condoleezza Rice. Current Gov. Jerry Brown (D-CA) is a clear favorite against both Republican challengers in the November general election.

South Dakota U.S. Senate race: Former Gov. Mike Rounds (R) has been the GOP frontrunner in the race to replace retiring Sen. Tim Johnson (D-SD), although state Rep. Annette Bosworth (R) has been getting a fair amount of attention in the five-person Republican primary. The winner will face Democratic businessman Rick Wieland, who was once a staffer for former Sen. Tom Daschle (D-SD)